Field of the Invention
The field of the present invention relates to hollow blades, in particular gas turbine blades, and more particularly to the moving blades of turbine engines, specifically the moving blades of a high pressure turbine.
Description of the Related Art
In known manner, a blade comprises in particular an airfoil extending in a longitudinal direction, a root, and a tip opposite from the root. For a moving turbine blade, the blade is fastened to the disk of a turbine rotor by means of its root. The blade tip is situated facing the inside face of the stationary annular casing surrounding the turbine. The longitudinal direction of the airfoil corresponds to the radial direction of the rotor or of the engine, with this being relative to the axis of rotation of the rotor.
The airfoil may be subdivided into airfoil sections that are stacked in a stacking direction that is radial relative to the axis of rotation of the rotor disk. The blade sections thus build up an airfoil surface that is subjected directly to the gas passing through the turbine. From upstream to downstream in the fluid flow direction, this airfoil surface extends between a leading edge and a trailing edge, these edges being connected together by a pressure side face and a suction side face, also referred to as the pressure side and the suction side.
The turbine having such moving blades has a flow of gas passing therethrough. The aerodynamic surfaces of its blades are used for transforming a maximum amount of the kinetic energy taken from the flow of gas into mechanical energy that is transmitted to the rotary shaft of the turbine rotor.
However, like any obstacle present in a gas flow, the airfoil of the blade generates kinetic energy losses that need to be minimized. In particular, it is known that a non-negligible portion of these losses (in the range 20% to 30% of total losses) can be attributed to the presence of functional radial clearance between the tip of each blade and the inside surface of the casing surrounding the turbine. This radial clearance allows a flow of gas to leak from the pressure side of the blade (zone where pressure is higher) towards the suction side (zone where pressure is lower). This leakage flow represents a flow of gas that does no work and that does not contribute to expansion in the turbine. Furthermore, it also gives rise to turbulence at the tip of the blade (known as the tip vortex), which turbulence generates high levels of kinetic energy losses.
In order to solve that problem, it is known to modify the stacking of the sections of the blade at the level of the blade tip, in order to offset the stacking towards the pressure side face, this offset preferably taking place progressively, being more pronounced for sections that are closer to the free end of the tip.
Blades of this type are referred to as blades with an “advanced blade top” or as blades with a “tip section offset”.
Furthermore, turbine blades, and in particular the moving blades of a high pressure turbine, are subjected to high temperature levels by the external gas coming from the combustion chamber. These temperature levels exceed the temperatures that can be accepted by the material from which the blade is made, thus requiring the blades to be cooled. Recently-designed engines have ever-increasing temperature levels for the purpose of improving overall performance, and these temperatures make it necessary to install innovative cooling systems for the high pressure turbine blades in order to ensure that these parts have a lifetime that is acceptable.
The hottest location in a moving blade is its tip, so cooling systems seek firstly to cool the top of the blade.
A wide variety of techniques have already been proposed for cooling blade tips, and mention may be made in particular to those described in EP 1 505 258, FR 2 891 003, and EP 1 726 783.
Consequently, it can be understood that the particular configuration that arises when using the “tip section offset” technique disturbs the performance and the effectiveness of conventional cooling systems in the tip zone of the blade.
Unfortunately, the top of a blade is always the hottest location of a moving blade, so it is essential for the “tip section offset” technique to be capable of coexisting with a cooling system that remains effective in order to conserve a lifetime for the part in this zone that is sufficient when subjected to high temperature conditions upstream.
It is found that those solutions are not compatible with the “tip section offset” technique.